#include <cstdio>
#include <set>
#include <queue>
#include <algorithm>
#include <climits>

using namespace std;
const int MAX_N = 5000;

int v[MAX_N + 5];

set <int> s;
queue <int> q;

bool f[MAX_N + 5];

int main () {
  freopen ("secv.in", "r", stdin);
  freopen ("secv.out", "w", stdout);

  int N;
  scanf ("%d", &N);
  for (int i = 1; i <= N; ++i) {
    scanf ("%d", &v[i]);
    s.insert (v[i]);
  }

  int my_max = 0;
  set<int> ::iterator it;
  set<int> ::iterator it2;
  for (int i = 1; i <= N; ++i) {
    it = lower_bound (s.begin(), s.end(), v[i]);
    v[i] = *it;
    my_max = max (my_max, v[i]);
    if (v[i] == 1)
      q.push (i);
  }

  int my_min = INT_MAX;
  while (!q.empty()) {
    int poz = q.front();
    q.pop();
    f[0] = true;
    int last = 0;
    int i;
    for (i = poz; i <= N; ++i) {
      if (v[i] == last + 1) {
        f[v[i]] = true;
        last = v[i];
      }
      if (f[my_max] == true)
        break;
    }
    if (f[my_max])
      my_min = min (i - poz + 1, my_min);
    for (i = 0; i <= my_max; ++i)
      f[i] = false;
  }

  if (my_min != INT_MAX)
    printf ("%d\n", my_min);
  else
    printf ("-1\n");
  return 0;
}